Cell Motility

Motility and migration of cancerous cells lead to metastases and the formation of secondary tumors. Cell motility is correlated with the metastases of the cells and has been studied using phagokinetics tracks [54, 55]. Several methods have been developed to record the phagokinetic tracks of cells [55-59]. One method, based on real-time observation of phagokinetic tracks of cells, has been developed, in which single live cells are passed over a layer of markers and ingest the markers and hence individual cells leave behind a trail of blank spots that automatically register the area that the cells traveled through [55]. These trails reflect the

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Figure 6. Tracks generated by MHC class of HeLa cells obtained using high-sensitivity fluorescence imaging [52]. The tracks of MHC class I were obtained using 20 nm FluoSphere beads labeled with PE-Fab, CML 100-Fab, and CML 30-Fab. Representative samples of the tracks obtained using PhyE-Fab (A) and CML 100-Fab (B) on cells and PhyE-Fab (C) on poly-L-lysine-coated microscope slides. Anomalous diffusion patterns were deduced from these tracks generated by linking the spots obtained at approximately 4-s intervals. Reprinted with permission from [52], P. Smith et al., Biophys. J. 76, 3331 (1999). © 1999, Biophysical Society.

phagokinetic tracks of cells. The advantage of this method is that it provides a rapid and automatic method for the study of cell motility that reflects both temporal and spatial information of the individual paths. The selection of markers for this method has been difficult because these markers have to be able to be uniformly coated on the surface, be ingested rapidly by cells, and endure long luminescence. Organic dyes are rapidly bleached and are unable to serve as markers. Au nanoparticles have served as the markers in this method for the study of phagokinetic tracks of cells. Due to the low-sensitivity detection means, larger Au nanoparti-cles (150 nm) have been used as markers for this method and hence it suffers severe drawbacks such as the lack of uniformity in both the film of Au nanoparticles and the sizes of Au nanoparticles. As ultrasensitive detection means advance, smaller Au and Ag nanoparticles (10-20 nm) will be used to circumvent these problems.

Recently, relatively photochemically stable luminescence probes, semiconductor QDs (CdSe/ZnS/SiO2) (4-8 nm), have been used to study the motility of live cells [12]. These QDs are stable and soluble under physiological buffer conditions and have been used as efficient light emitters for a variety of applications [27-34]. Thin layers of QDs were deposited on a collagen-coated tissue culture substrate and then the cells were seeded on the substrate [12]. The cells and QDs were monitored in real time by using confocal fluorescence microscopy or multiphoton microscopy. As the cells migrated and ingested the QDs, the cells freed the immobilized QDs and created a trail of blank spots that clearly revealed the cell migration paths (Fig. 7). Thus, the migratory paths of tumors and normal cells were assayed simultaneously at millisecond temporal resolution. This allowed the cell motility to be calculated and the metastases of the cells to be studied. QDs have been conjugated with a wide variety of molecules for the detection of targeting molecules. Moreover, the emission spectra of QDs are size dependent [27-34]. These unique features of QDs have empowered

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HUMAN MAMMARY EPITHELIAL CELLS (MC F 10A)

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Figure 7. Migratory paths of the tumor cells determined by differential interference contrast (DIC) and fluorescence confocal microscopy [12]. Migratory paths of the tumor cells could be determined by engulfment of QDs. Human mammary epithelial tumor cells, MDA-MB-231 (A-F), and nontumor cells, MCF 10A (G-L), were grown on collagen that had been coated with a thin layer of silanized, water-soluble, red fluorescent semiconductor QDs. The merged images (A, D, G, J) show that the cells and the layer of QDs beneath the tumor cells (B) and nontumor cells (H) were fairly uniform. After 24 h, (i) the large clearings in the QD layer were observed around the tumor cells (D, E), but not around the nontumor cells (J, K). (ii) Tumor cells filled with QDs offered intensive bright fluorescence (E), whereas nontumor cells did not provide fluorescence (K). (iii) Some tumor cells were crawling into the underlying collagen and these tumors were less distinct in the DIC image (F). The images were collected using fluorescence confocal microscopy to examine QDs (B, E, H, K) and using DIC optical microscopy to visualize the cells (C, F, I, L) where the images were collected at the optical section to show the layer of QDs. The scale bar was 200 fxm. The results show that the tumor cells are more motile. Reprinted with permission from [12], W. Parak et al., Adv. Mater. 14, 882 (2002). © 2002. Wiley-VCH.

multicomplex analysis in buffer solution [60, 61]. This offers the possibility of the measurement of cell motility and the detection of a group of specific proteins simultaneously in real time using QDs. This also makes it feasible to study cell motility in three dimensions if the different sizes of QDs in a vertical gradient are prepared in extracellular matrix media. Studies of this kind will surely advance our understanding of cell motility and metastases.

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